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Experts say younger women should still get vaccinated against COVID-19 even if they experience mild disruptions to their menstrual cycle. McKinsey Jordan/Stocksy United Photo
  • Researchers say COVID-19 vaccines may cause minor, temporary changes in menstrual cycles.
  • Experts say these mild disruptions should not discourage someone from getting a COVID-19 vaccine.
  • They also note that other factors, including stress, can cause disruptions to a menstrual cycle.

The COVID-19 vaccine could lead to mild, temporary changes in the length of your menstrual cycle, according to a study published today in the journal Science Advances.

Nonetheless, experts say vaccines remain one of the best ways to avoid serious illness from the novel coronavirus.

The study’s researchers, led by Kate Clancy, Ph.D., an anthropologist and professor at the University of Illinois Urbana-Champaign, looked at responses by 35,000 premenopausal and postmenopausal individuals about their menstrual cycle in the weeks after receiving a COVID-19 vaccine.

None of the participants had been diagnosed with COVID-19. Researchers excluded those between 45 and 55 to avoid confusion with irregular periods due to premenopause or postmenopause.

The researchers reported that:

  • 42% of menstruating individuals reported a heavier flow after receiving the vaccine. Some experienced this within the first 7 days. Others did so between 8 and 14 days after receiving the vaccine.
  • Nearly 44% reported no changes
  • 14% reported a mix of no change or a lighter flow

Other findings included:

  • Respondents who had experienced a pregnancy were most likely to report heavier bleeding.
  • Most non-menstruating premenopausal respondents on hormonal treatments experienced breakthrough bleeding.
  • More than 70% of respondents using long-acting reversible contraception, such as an IUD, experienced breakthrough bleeding.
  • Slightly more than 38% of those undergoing gender-affirming hormone treatments reported breakthrough bleeding.

The following groups reported heavier menstruation:

  • Non-white, Hispanic/Latinx respondents
  • Older adults
  • Those who experienced fever or fatigue as a side effect of the vaccine
  • People with endometriosis, menorrhagia, fibroids, and other reproductive issues

The scientists said they believe the menstrual changes are usually short-term.

Earlier vaccine trials did not include menstrual issues as a potential side effect. Therefore, researchers said it wasn’t tracked or was ignored or dismissed.

Many respondents suggested that their doctors were often dismissive of their concerns. Some clinicians didn’t understand how a vaccine could cause menstruation changes.

However, the researchers pointed out that other vaccines, such as those for typhoid, hepatitis A and B, and HPV, have been associated with menstruation changes.

Some experts say an uptick in immune-related inflammatory pathways could be the cause of nonhormonal changes.

“The vaccine is designed to affect our immune system and our immune system affects our menstrual periods. I think there’s good biological plausibility that that could be happening, meaning there are biological explanations to explain some connection between vaccines and menstrual periods. However, it has not been studied,” Dr. Jessica Tarleton, MPH, explained in an article on the Medical University of South Carolina website.

These findings were also the result of self-reported surveys.

“It’s an interesting report, but they are anecdotal findings. However, many extraneous variables weren’t considered, such as how many women had polyps, fibroids, were stressed, postmenopausal with new onset bleeding, or were in the early stages of pregnancy,” said Dr. Kecia Gaither, MPH, double board-certified in OB/GYN and maternal fetal medicine and director of Perinatal Services/Maternal Fetal Medicine at NYC Health and Hospitals/Lincoln.

“I think a more stringent analysis would be needed to make a connection,” she told Healthline.

A study published in the journal Obstetrics and Gynecology earlier this year did find similar results. Researchers reported less than a 1-day increase in the length of the menstrual cycle around the time of immunization. The effect was temporary and within the range of normal variation.

This study gathered information from 4,000 women ages 18 to 45 via a fertility tracking app. The data compared three menstrual cycles before the vaccine and three after inoculation. For non-vaccinated women, the app used six consecutive cycles.

The women who had the vaccine had an average increase in cycle days of 1 day per dose (but not necessarily bleeding days.) There was a 2-day increase for women who received two vaccines in one cycle. As the time since the vaccine increased, menstrual cycles decreased to pre-vaccine levels.

Other factors could be affecting menstruation cycles.

“Less severe, short-term stressors can and do influence menstrual cycling, established in over 40 years of cycle research,” said Dr. Taraneh Shirazian an associate professor and director of the Division of Global and Community Women’s Health in the Department of Obstetrics & Gynecology at NYU Langone Health.

“The vaccine doesn’t have any long-term consequences, but getting COVID disease will likely disrupt the menstrual cycle much more so,” she told Healthline. “Changes could be linked to infection, weight loss/gain, and new medications. This information helps us understand that minor cycle changes are possible and common in response to vaccination, so women shouldn’t be surprised if their cycles change slightly.”

Menstrual cycles begin on the first day of your period and end on the first day of your next period.

Menstrual cycles can change depending on your age. A typical cycle can last anywhere between 24 and 38 days. When you first start menstruating, cycles longer than 38 days are typical. They usually become more regular within three years of starting your period.

During your 20s and 30s, cycles are typically regular and last between 24 and 38 days. Once you reach your 40s, your body starts to transition to menopause and you might have regular periods for a while and then stop for several months. You might also notice your flow changes – either lighter or heavier than your typical period.

“Monitor for any cycle change and contact your physician if it persists. Interestingly [in the study], postmenopausal patients reported breakthrough bleeding, which could support the idea that spotting with the vaccine is not associated with fertility. Instead, it could be just the result of a new ‘stress’ on the reproductive system,” said Shirazian.

In general, according to Penn Medicine, women should talk to their physician if:

  • If you haven’t had a period for 90 days.
  • Your period suddenly becomes irregular.
  • You have a period more often than every 21 days.
  • You have a period less often than every 35 days.
  • Your period lasts longer than 7 days.
  • Bleeding or spotting happens between periods.
  • If you experience pain during menstruation.